Introduction: Long known in the recording and mastering industries for their professional gear, Weiss Engineering of Switzerland more recently made a successful transition into the hifi and audiophile markets. Its CD transports, amplifiers and D/A converters named after various heroes and other beings from Greek mythology quickly established themselves as objects of Swiss-built technological desire that delivered the sonic goods as well.

The newest products from Weiss, perhaps befitting their intended application in computer-based digital music systems, are instead designated by more mundane acronyms and numerals.

This 202 series comprising the INT 202 FireWire interface, DAC 202 digital-analogue converter and ATT 202 passive attenuator represents Daniel Weiss’ latest thinking on how to make digital work well in the home environment.
The INT 202 brings conventional DACs—those lacking USB or FireWire interfaces—into the world of computer audio. Key features include absolute phase selection, precision attenuation and muting. One can thus repurpose an existing DAC, retire a preamplifier and armed with the INT 202’s optional remote control reign over the emerging world of high-resolution computer audio from the comfort of one’s favorite listening chair.

Unlike some competing products from firms that share the professional market heritage, the 202 Series components I’ve been evaluating—the INT 202 and DAC 202—reveal considerable insight into how consumers will actually use them. This is manifested not only by their high-end performance but also creature features that make one smile with their cleverness. And, these Weiss designs tackle at a fundamental level the key issues of high-resolution digital audio.

Regarding those, one ongoing discussion in digital audio circles revolves around jitter and whether one type of interface (USB or FireWire in this case) is inherently better. As the herd thunders off in the direction of USB interfaces and DACs with mixed but gradually improving results, Daniel Weiss clearly believes that continued refinement of devices connected to the digital bitstream via FireWire is the way forward at least for the time being.

Regardless of whether one uses USB or FireWire for data transfer and what particular mode of operation is employed, what one does with the data once it arrives is the real problem. The main difference in audio quality offered by various products, from my observations to date, is not so much dependent upon how, where and what type of jitter is generated—it’s present in every digital audio device and interface—but how the designer deals with it. This is the real competition between designs. Subtle variations in data timing a.k.a. jitter take many forms and are inherent in digital audio. Jitter along with quantization noise are ineluctable facts of life in the realm of digital music. With this in mind and based upon months of listening to various USB-based computer interfaces and DACs, I was quite interested to hear how a FireWire-based digital interface promising ultra-low jitter might sound by comparison.

Main features: The Weiss INT 202 connects both DIY music servers—repurposed personal computers—and purpose-built equivalents with external DACs. Thus the primary function of the INT 202 is to convert linear pulse code modulation (PCM) digital audio data streams arriving from the computer over FireWire network communications protocol and hardware to the AES/EBU and S/PDIF digital audio interface formats commonly accepted by consumer DACs.
Secondarily and equally importantly, the INT 202 regenerates clean low-jitter digital clocks so that what’s passed to the external DAC will not contribute to various unpleasant distortions later on.

Sampling rates of 44.1, 48.0, 88.2, 96, 176.4 and 192kHz are supported in both 16 and 24-bit word lengths. An optional infrared remote control allows fine adjustment of a software-based digital level attenuator as well as polarity reversal and muting. The INT 202 can also be used without remote control, for example with products such as the two-channel Berkeley Audio Design Alpha DAC which already have attenuation and phase reversal functions but lack a computer interface. In this case the INT 202 defaults to 0dB attenuation or unity gain but still provides format conversion, clock regeneration and bit-perfect transfer of information from the audio server.

For connecting to a DAC, the INT 202 unit has a generous complement of digital output connectors: 2 x 110Ω XLR for AES-EBU and 2 x 75Ω RCA for S/PDIF. Best results in my system were achieved with the AudioQuest Eagle Eye for S/PDIF and similar Raven for XLR. Weiss Engineering’s Chiron unshielded XLR cable was a close second. There are also two 6-pin FireWire receptacles for data, which can further supply power in case it’s available from the source device.

On my Toshiba laptop, an AudioQuest 1394c cable with that rather small 4-pin connector at the computer end and the much larger 6-pin plug at the interface worked without any problems. Because laptops generally don’t (and probably shouldn’t) supply power with their FireWire ports, one instead uses a diminutive power supply module supplied by Weiss, which slips onto the female end of a power cord.

FireWire/IEEE 1394 Interface: So why use FireWire in this age of ubiquitous USB? The FireWire technology originated in 1986 as Apple Computer’s solution for a very high-speed serial isochronous peer-to-peer communication network between computers and peripheral devices. The IEEE eventually developed a comprehensive standard, IEEE 1394, which incorporates the technical contributions of several additional companies (among them Sony, Texas Instruments, IBM, Digital Equipment Corporation and INMOS) who deploy the technology under various brand names.
With IEEE 1394, its ability to guarantee bandwidth and quality of service is ideal for streaming large chunks of digital data without error or long latency. On most higher-spec personal computers (e.g. Apple, Sony, Toshiba, Lenovo, Dell and HP), IEEE 1394 is usually available although less commonly now than a few years back.

Although USB is universal—always a big plus—occasionally one may find that some USB ports are created more equal than others with respect to data transfer capabilities and ability to power external devices. While isochronous master-slave communication is also supported, USB 2.0 and 3.0 are just now moving into the high bit-rate digital audio regime. There also remain important issues such as protecting and isolating downstream equipment from the numerous noise sources rampant in most computers. Then there’s the substantial electromagnetic interference (EMI) that radiates at considerable distance from their switching power supplies disliked in audiophile circles.

By comparison, FireWire is a more mature and robust technology and some of the problems USB is now undergoing have already been sorted out. FireWire and IEEE 1394 enjoy broad adoption by professional recording studios, video camera manufacturers and post-production houses. Its ability to transport high-resolution multi-channel audio as well as HD video make it a preferred protocol for many applications in the world of content creation, editing and mastering.

Compared to the cheaper and easier implemented USB interface, IEEE 1394 is essentially a distributed peer-to-peer networking technology, meaning it can operate at full speed with little intervention by a central CPU. In other words, it’s really smart. FireWire devices can communicate between themselves at the hardware level more or less autonomously. As a result, this approach offers inherently low latency, which is beneficial for many applications.

Dancing with jitterbugs: In essence, jitter comprises timing errors that are inherent in the digital representation and processing of audio and video. Jitter is found in several areas: communications interfaces (USB or FireWire) between digital components; the word or sample clocks of A/D and D/A converter; as well as the bit patterns of the data themselves. These all create various forms of jitter. Although evanescent in nature, jitter gives rise to a variety of audible artifacts. It’s the nature of the beast and it’s a big beast.

On the input side, the Weiss design uses isochronous FireWire communications handshaking. The INT 202 as receiving device becomes bus master and thus has complete control over the timing and transfer of information from the source, a computer in this case. This part is relatively straightforward and reduces certain forms of jitter to a considerable degree.

On the output side, consider that AES and S/PDIF are essentially continuous streaming protocols with embedded clock information.

To the extent that the subsequent D/A conversion is coupled to the clocking in the audio data, removal of jitter at this stage is especially critical. The interface must ideally deliver low-jitter clocking information for the outgoing digital audio bit streams and this process can become rather complex.
To orchestrate all this, Weiss built the INT 202 around a specialized and powerful very large-scale integrated circuit (VLSI), the TC Communications TCD2220 DICE (digital interface communication engine). Inside this chip, there’s a complete 32-bit ARM reduced instruction set computer (RISC) microprocessor, I²S and AES ports; an IEEE 1394a controller; and a specialized JET (jitter-elimination technology) phase-locked loop (PLL) that hybridises analogue and digital technology. The JET PLL regenerates the timing clocks as it attenuates pass-through of jitter. Built for applications much more demanding than two-channel home audio, DICE can handle a maximum of 32 input and output channels simultaneously - or up to 16 channels at 192kHz.

What’s a PLL and how does it work? Conventional analogue PLLs are essentially electronic flywheels that reduce jitter by smoothing out timing variations. To understand the basic principle, consider a human-powered potter’s wheel. Once up to speed, an occasional additional kick of varying energy and timing keeps everything rotating at a reasonably constant velocity. The variations in the timing of each input kick are damped by the rotational mass of the flywheel. That’s the basic idea.

In the case of electronic systems, if the variability in the input data clock changes relatively slowly—say less than 100Hz or so—the audio artifacts created by such slow jitter are not likely to be readily audible. The resulting distortions are usually very close to the primary frequency, which masks their presence. In this case, the output of the simple PLL tracks the input clock as it drifts in a benign fashion. However, if the PLL detects jitter periodicity higher than the nominal knee or corner frequency of 100Hz, an associated crystal-controlled high-precision oscillator becomes increasingly dominant in terms of generating outgoing clock data. This works well enough for many applications from the days of Red Book CDs but in the domain of high-resolution digital audio and ever-rising consumer expectations, its shortcomings are increasingly apparent.